The Rule of 72: A Quick Mental Math Trick for Investors

Mental Math for Investors

While our Compound Interest Calculator provides precise figures, sometimes you just want a quick estimate. Enter the Rule of 72, a simple formula used to estimate the number of years required to double your invested money at a given annual rate of return.

How to Use It

The formula is incredibly simple: divide the number 72 by your expected annual interest rate. The result is the approximate number of years it will take for your investment to double.

Example 1: If you expect a 6% annual return, divide 72 by 6. It will take approximately 12 years for your money to double (72 / 6 = 12).

Example 2: If you expect a 9% annual return, divide 72 by 9. It will take approximately 8 years for your money to double (72 / 9 = 8).

Limitations

The Rule of 72 is an approximation, not an exact calculation. It is most accurate for interest rates between 6% and 10%. For higher or lower rates, the estimate becomes slightly less precise, but it remains a valuable tool for quick mental calculations and comparing different investment options.